Conventionally, the HDMI (High-Definition Multimedia Interface) is known as a standard for inter-apparatus transmission of video signals and audio signals. In a transmission system conforming to the HDMI standard, video signals and audio signals can be transmitted from the transmitting apparatus as a source device to the receiving apparatus as a sink device with no compression.
In recent years, an image resolution of a video image source is becoming higher and a 3D video image is becoming more widespread. Under such circumstances, it is required the non-compression transmission between the apparatuses to speed up its transmission speed.
In order to meet such requirement, it is considered to apply the optical transmission technique that has conventionally been used for a long distance transmission or a high-speed transmission between industrial devices such as a router, a supercomputer and the like to the transmission between consumer devices. In addition, being different from cables for electric power transmission, an optical fiber (an optical cable) used as an optical transmission path never emit electromagnetic noise, and therefore is expected to produce an effect of suppressing the electromagnetic noise.
Meanwhile, in the HDMI standard at present, the function of detecting cable connection between apparatuses preceding the initiation of the inter-apparatus transmission, i.e., what is called a hot plug function (hot plug detect (HPD)) is provided for. In the case where an optical transmission system is applied to a video signal transmission system between consumer devices, it is favorable that a function similar to the hot plug function be also added to such optical transmission system for the purpose of securing the backward compatibility for the HDMI.
Other than the purpose that the backward compatibility for the HDMI should be secured, the hot plug function is useful for the optical transmission system for following reasons.
In optical transmission systems, semiconductor lasers are usually used as light emitting elements. As for an apparatus that uses a semiconductor laser, intensity of laser light emitting outside the apparatus is regulated for the purpose of securing safety of the eyes.
One of the methods for meeting the regulation is to drive a semiconductor laser such that the intensity of the laser light emitted from the light emitting element should be equal to or smaller than the restricted intensity at any time. Naturally, in this case, tolerable light loss at the optical transmission path becomes small.
Accordingly, coupling loss between the light emitting element (semiconductor laser) or the light receiving element (photodetector) and the optical transmission path (optical fiber) have to be maintained low. Consequently, high assembly precision is required for the optical system, which disadvantageously brings about an increase in costs.
Another method for meeting the regulation is to control the semiconductor laser (light emitting element) such that the semiconductor laser stops when the light emitting element and the light receiving element are not connected with each other via the optical transmission path, and that the semiconductor laser drives when they are connected with each other. According to this method, when the light emitting element and the light receiving element are not connected via the optical transmission path with each other, the laser light is not emitted outside. Therefore, the intensity of the laser light output from the semiconductor laser is not restricted by the regulation described above.
However, this method requires a system for detecting that the light emitting element and the light receiving element are connected via the optical transmission path with each other.
For the reasons described above, realization of the optical transmission system provided with the hot plug function has been expected.
Patent Literature 1 (JP2004-350155A) discloses a cable connection detecting method suitable for optical transmission systems. FIGS. 20 and 21 exemplarily show the scheme disclosed in Patent Literature 1. With reference to FIGS. 20 and 21, an optical communication apparatus 910 including a light emitting element (a laser 914) and an optical communication apparatus 920 including a light receiving element (a photodetector 921) are connected via an optical fiber cable 930 with each other. The optical fiber cable 930 includes an optical fiber 932, and further includes conducting wires 931 and 933.
The optical communication apparatus 910 includes an energizing circuit 915, an impedance element 911, and a monitor 912 that monitors the energizing state of the energizing circuit 915. The optical communication apparatus 920 includes an energizing circuit 916 and an impedance element 922.
As shown in FIG. 20, when the optical communication apparatus 910 and the optical communication apparatus 920 are connected via the optical fiber cable 930 with each other, the impedance element 911, the energizing circuit 915, the conducting wire 931, the energizing circuit 916, the impedance element 922, and the conducting wire 932 are brought into conduction state. In this state, the monitor 912 detects impedance which is equal to the impedance when the impedance element 911 and the impedance element 922 are connected in parallel. In this case, the output control unit 913 determines that it is in the normal connected state, and starts light emission of the laser 914 at a prescribed intensity.
On the other hand, as shown in FIG. 21, when the optical communication apparatus 920 and the optical fiber cable 930 are not connected with each other, the conducting wires 931 and 933 and the energizing circuit 916 are not electrically connected with each other. Accordingly, the monitor 912 detects impedance that is equal to the impedance of the impedance element 911 solely. In this case, the output control unit 913 determines that it is in the disconnected state, and stops the light emission of the laser 914.
Thus, the scheme disclosed in Patent Literature 1 provides the conducting wires (i.e., a conductive material) around the optical transmission path, to thereby make it possible to detect whether or not the apparatuses are connected at the both ends of the optical transmission path based on the impedance.